The long term objective to this project is to determine cell and molecular changes after irradiation leading to mammary tumor development. Using in vivo/in vitro models, previous studies in this laboratory have identified phenotypic alterations in mouse mammary epithelial cells which precede the development of mammary tumors in Balb/c female mice. These studies have also confirmed the preneoplastic nature of the cells exhibiting these phenotypic changes. Over the last grant period, these models were used to investigate the involvement of specific alterations in oncogenes and tumor suppressor genes in these phenotypic changes. These data strongly suggest that alterations in the tumor suppressor gene, p53, are involved as early events in the process of neoplastic development following radiation exposure. Research over this next grant period is designed to test this hypothesis. Specifically, the aims of this research are to: 1) Determine whether mutations in p53 are early events in the time course of preneoplastic development after irradiation. Studies will examine whether mutations in p53 are consistently associated with preneoplastic cells induced by irradiation, whether these mutations are detected early after irradiation, and whether such mutations are observed in vivo in ductal dysplasias. 2) Determine the functional role of p53 in neoplastic progression of murine mammary cells. These studies will examine effects of altered p53 on cellular phenotype and cell function. 3) Determine whether alterations in p53 are preceded by earlier radiation induced genetic alterations. These studies will examine other early genetic alterations in irradiated mammary epithelial cells.